1. Field of the Invention
The present disclosure relates to an adsorbent for trapping a radioactive iodine gas generated in a process of oxidizing a nuclear fuel at a high temperature after use and a method of manufacturing the same, and more particularly, to a radioactive iodine gas adsorbent which is consisted of bismuth as a main component to exhibit an excellent radioactive iodine gas trapping capability and an excellent thermal stability after trapping, and a method of preparing the same.
2. Discussion of Related Art
A nuclear fuel is recycled through a wet or dry reprocessing method after use, and volatile nuclides are volatilized through detailed processes such as nuclear fuel bundle dismantlement, cutting, removal of cladding, and a high temperature oxidizing process, which correspond to a pre-treatment process of the recycling. Representative volatile nuclides are H-3, C-14, Kr-85,I-129, etc. Among them, I-129 is a representative long half life nuclide having a half life of 1.57×107 years, and I-129 included in a waste is easily dissolved into an I− ion when in contact with underground water. Since the I− ion dissolved in the underground water does not easily react with surrounding soil, it is highly probable that it inflicts a harm upon an ecosystem according to flow of the underground water. When a radioactive iodine gas is absorbed into a human body and accumulated in a thyroid, it can cause a severe harm, and therefore should be trapped in a safe way to prevent emission to the air. According to the U.S. environmental radioactivity standard, an annually allowed human radiation dose is 25 mSv, and annually, an iodine decontamination factor of an adsorbent should be at least 167 to trap an I-129 off-gas generated from a nuclear fuel in accordance with the standard after use.
Various methods for trapping an I-129 gas generated in a nuclear fuel treating process after use are studied. As a wet processing method, a method of scrubbing the nuclear fuel after use in a Hg(NO3)2, NaOH, or HNO3 solution has been studied, and as a dry processing method, a method of trapping a gas using activated carbon, resin, silver-containing silica, or zeolite has been studied and widely used to trap an iodine gas. Today, a study for trapping an iodine gas at 150 to 200° C. using a bead-type silver exchange zeolite (AgX) is being conducted by Korean Atomic Energy Research Institute (KAERI), and iodine is physically trapped in a zeolite pore, or reacts with Ag to form a compound called AgI.
The silver-containing zeolite is used today as a basic technology to remove an I-129 gas due to various advantages such as efficiency and reproductivity for trapping an iodine gas. However, since silver is expensive, a processing cost is increased in the removal of a large amount of I-129, and silver is also a toxic element strictly regulated by the U.S. Environmental Protection Agency (EPA). Moreover, since many parts of the trapped iodine are physically trapped in a zeolite pore, thermal stability of the trapped iodine is decreased, and when the zeolite having completed trapping is thermally treated to long-term disposal in order to form a durability-strengthened wasteform, sintering can be performed by performing heat treatment with a high temperature of 1,000° C. or more.
Accordingly, as an adsorbent for trapping an I-129 off-gas generated from a nuclear fuel after use, development of a new adsorbent which is economical, has excellent iodine trapping performance and excellent thermal stability after trapping, and facilitates sintering through thermal treatment of the adsorbent becomes a target of the main object, and there is an attempt for a research on the development (refer to JP2012-215551A), but the attempt is incomplete yet.